Re: [Lsf-pc] [LSF/MM/BPF TOPIC] Removing GFP_NOFS

[Jan Kara <jack@suse.cz>]

On Fri 05-01-24 13:13:11, Viacheslav Dubeyko wrote:
> 
> 
> > On Jan 5, 2024, at 12:17 AM, Matthew Wilcox <willy@infradead.org> wrote:
> > 
> > This is primarily a _FILESYSTEM_ track topic.  All the work has already
> > been done on the MM side; the FS people need to do their part.  It could
> > be a joint session, but I'm not sure there's much for the MM people
> > to say.
> > 
> > There are situations where we need to allocate memory, but cannot call
> > into the filesystem to free memory.  Generally this is because we're
> > holding a lock or we've started a transaction, and attempting to write
> > out dirty folios to reclaim memory would result in a deadlock.
> > 
> > The old way to solve this problem is to specify GFP_NOFS when allocating
> > memory.  This conveys little information about what is being protected
> > against, and so it is hard to know when it might be safe to remove.
> > It's also a reflex -- many filesystem authors use GFP_NOFS by default
> > even when they could use GFP_KERNEL because there's no risk of deadlock.
> > 
> > The new way is to use the scoped APIs -- memalloc_nofs_save() and
> > memalloc_nofs_restore().  These should be called when we start a
> > transaction or take a lock that would cause a GFP_KERNEL allocation to
> > deadlock.  Then just use GFP_KERNEL as normal.  The memory allocators
> > can see the nofs situation is in effect and will not call back into
> > the filesystem.
> > 
> > This results in better code within your filesystem as you don't need to
> > pass around gfp flags as much, and can lead to better performance from
> > the memory allocators as GFP_NOFS will not be used unnecessarily.
> > 
> > The memalloc_nofs APIs were introduced in May 2017, but we still have
> > over 1000 uses of GFP_NOFS in fs/ today (and 200 outside fs/, which is
> > really sad).  This session is for filesystem developers to talk about
> > what they need to do to fix up their own filesystem, or share stories
> > about how they made their filesystem better by adopting the new APIs.
> > 
> 
> Many file systems are still heavily using GFP_NOFS for kmalloc and
> kmem_cache_alloc family methods even if  memalloc_nofs_save() and
> memalloc_nofs_restore() pair is used too. But I can see that GFP_NOFS
> is used in radix_tree_preload(), bio_alloc(), posix_acl_clone(),
> sb_issue_zeroout, sb_issue_discard(), alloc_inode_sb(), blkdev_issue_zeroout(),
> blkdev_issue_secure_erase(), blkdev_zone_mgmt(), etc.

Given the nature of the scoped API, the transition has to start in the
leaves (i.e. filesystems itself) and only once all users of say
radix_tree_preload() are converted to the scoped API, we can remove the
GFP_NOFS use from radix_tree_preload() itself. So Matthew is right that we
need to start in the filesystems.

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR